The invention relates to a beam landing correction arrangement in a video display apparatus.
The displayed image in, for example, a direct view video display or in a projection video display having a cathode ray tube (CRT), may suffer from beam landing location errors such as geometrical and misconvergence errors. It is known to correct such errors for a CRT using a digital dynamic convergence arrangement. Correction data stored in a memory are applied via a digital-to-analog (D/A) converter and a power amplifier to, for example, an auxiliary convergence winding. The amount of correction may vary dynamically in a given deflection cycle, in accordance with the location of the beam on the display screen.
In a video display, embodying an inventive feature, correction data are stored in a first non-volatile memory that retains the correction data even when it is not energized. Upon power turn on, for example, the correction data stored in the non-volatile memory are read out and stored in a volatile memory. During each deflection cycle, the data stored in the volatile memory are successively read out and applied via a D/A converter to an auxiliary convergence winding;
whereas, the data from the non-volatile memory need not be read out during the deflection cycle. Because the data is retained in the non-volatile memory, the volatile memory need not retain the data when the video display is turned off. By using such memory hierarchy, the correction data is accessible via the faster volatile memory and is retained, when the display is turned off, using the slower, non-volatile memory.
At the factory, for example, factory adjusted correction data are stored in the non-volatile memory. The factory adjusted correction data are unique to each set of the same model to compensate for production tolerances. Re-adjustment of the correction data may be required after the set has been relocated to a geographical location having a different value of the earth magnetic field from that existed, during factory set up. The readjustment of the correction data may be obtained with an automatic alignment system using photo sensors.
Non-transient alteration of the correction data may occur in the non-volatile memory, as well as in the volatile memory, because of energy released in the event of a CRT arc discharge. The non-transient data alteration might occur when the arc discharge occurred simultaneously with the reading out of the correction data from the non-volatile memory. Whereas, no such data alteration has been observed when the data read out process and the arc discharge do not occur simultaneously. It may be desirable to substitute in each memory the data containing error with valid data free of data error.
In carrying out an inventive feature, each convergence data word includes a parity bit derived by check summing the data in the word that is read out of the volatile memory. The parity bit is used to sense data bit error in the read out data. A parity checking detector is used to calculate the parity bit using the present read out data bits from the volatile memory. When a parity error is detected, the data stored in a second non-volatile memory are automatically read out and transferred to the volatile memory and to the first non-volatile memory. The second non-volatile memory may contain a duplicate of the correction data stored in the first non-volatile memory, during factory set up.
Unlike the correction data in the first non-volatile memory, the data in the second non-volatile memory are parity error free. This is so because, the data are not read out from the second non-volatile memory, during power up procedure so read out and the release of arc discharge energy do not coincide. Thereby, advantageously, the parity error containing data stored in the first non-volatile memory are substituted with the data, provided by the second, non-volatile memory, that are free of parity error.
A video display deflection apparatus, embodying an inventive feature which generates a deflection field in a cathode ray tube to vary a beam landing location of an electron beam of the cathode ray tube. A first memory space containing beam landing error correction data is provided. The correction data are read out of the first memory space, during a given deflection cycle. The correction data are applied to a deflection field generating arrangement for varying the deflection field by a variable amount that varies in accordance with the varying beam landing location. A second memory space containing beam landing error correction data is provided. The correction data are read out from the second memory space and stored in the first memory space, during a mode set up procedure. A third memory space containing beam landing error correction is provided. A bit error detector detects data bit error in the data stored in one of the first and second memory spaces. A controller is used for reading out beam landing error correction data of the third memory space and for storing the read out data of the third memory space in one of the first and second memory spaces. The data bit error containing data are replaced after the data bit error has been detected.